Planographic printing plates typically contain a substrate having a hydrophilic coating thereon, with oleophilic or ink-receptive image areas subsequently being produced directly on the hydrophilic surface. Various light-sensitive materials can be utilized to prepare such image areas, e.g. diazo resins, silver halide emulsions, cinnamic acid esters, etc.
The system having specific utility herein involves the utilization of silver salt diffusion transfer principles. Procedures for preparing photographic images of silver by diffusion transfer principles are generally well-known in the art. Typically, an image-wise exposed silver halide emulsion is contacted with a processing composition containing a developing agent to reduce the exposed silver halide to metallic silver in the emulsion and a silver halide solvent capable of providing a soluble silver complex with the unexposed silver halide. An image-wise distribution of this silver complex is then transferred to a superposed silver-receptive stratum where the silver complex is reduced to metallic silver to impart a silver transfer image to the stratum.
Diffusion transfer has been employed in connection with photolithography by using a hydrophilic material as the silver receptive stratum (to thereby provide the background of a printing plate). The silver image is then used to generate an ink-receptive printing surface on the hydrophilic background. In such a system, the hydrophilic silver-receptive stratum of the receptor sheet typically contains catalytic silver-precipitating nuclei dispersed in a macroscopically continuous vehicle or matrix. One such matrix which has been used is colloidal silica, the silver being deposited image-wise on the silica surface to thus provide the ink-receptive areas of the plate. The deposited silver can be treated, as is taught in U.S. Pat. No. 4,204,868, to render same sufficiently oleophilic to be useful in lithographic printing.
Colloidal silica has conventionally been utilized as the background area, i.e. the hydrophilic area of a printing plate, because silica optimizes the hydrophilicity thereof, most organic materials being less hydrophilic than silica and therefore tending to produce a printing plate which may scum or accept ink in the background areas more readily than those wherein silica is utilized. However, utilizing colloidal silica as the matrix can provide difficulties. First of all, a colloidal silica surface is typically not strong enough to withstand the pressures and abrasive forces exerted by the printing press rollers, thereby limiting the plate in terms of its press life. Secondly, a colloidal silica matrix may not allow sufficient silver deposition on the surface thereof to provide an ink receptive image area capable of extended press runs.
One suggested approach for utilizing silica and yet eliminating or reducing the difficulties mentioned above relates to the concept of using colloidal silica particles of two different sizes, the average particle size of one of the colloidal silicas being about 2.5 times that of the second colloidal silica. Such an improvement provides a tougher hydrophilic silica background capable of extended press runs, and because of the better compaction of the silica using this process, sufficient silver deposition on the silica surface is provided so as to afford a more durable ink-receptive image area.
While the foregoing solved the difficulties encountered with prior art structures, nevertheless, sufficient silver deposition had to be maintained in order to provide proper adhesion of the silver image area to the background, and to provide abrasion resistance to afford extended press life to the plate. With silver costs being what they are today, it would of course be extremely desirable to allow a reduction in the silver requirements without reducing the effectiveness of the resultant plate.
Surprisingly, I have now found that the addition of hereinafter-defined dry particles, such as silica powders, to the above-referenced colloidal silica lattice allows for a significant, i.e., about fifty percent, reduction in silver usage.
In British Pat. No. 1,419,511, there is disclosed a lithographic proofing receiver and lithographic plate which is taught to be manufactured by the use of fumed silica particles, or other inert particles, which have a larger diameter than colloidal silica particles. However, the image-receiving layer therein is formed using polyvinyl alcohol as a binder. Because colloidal silica has a hardening effect on the polyvinyl alcohol, patentees therein include the fumed silica or other inert materials. This is taught to reduce the effect of the colloidal silica on the polyvinyl alcohol. Furthermore, if the structure containing the polyvinyl alcohol is to be utilized on a lithographic press, patentees teach the necessity of the incorporation of the hydrolysis product of tetraethylorthosilicate, or another tetraalkylorthosilicate. The present invention has greater press latitude than the structure of the reference because the polyvinyl alcohol is not utilized herein.